Lancenigo di Villorba (TV), Italy
You treated about titrimetry of polyprotic acids.
You stated that :In polyprotic acids, the last equivalence point the sharpest on a titration curve.
You get reason.
Now, I start from a diprotic acid which i will define H2X like a WEAK DIPROTIC ACIDIC SPECIES able to ionize in aqueous solutions like it happen by following equilibria
i) H2X(aq) + H2O(aq) --- HX-(aq) + H3O+(aq)
Ka1 = |HX-| * |H3O+| / |H2X|
ii) HX-(aq) + H2O(aq) --- X--(aq) + H3O+(aq)
Ka2 = |X--| * |H3O+| / |HX-|
So, once you began ACID/BASE TITRIMETRY, you began addiction of chemical equivalent of Strong Base (e.g. usually NaOH) to
Title-Defined aqueous solution of Acidic Species H2X.
So, Acidic Species H2X exists besides its former derivative, e.g. HX- coming from former Chemical Equilibrium Relationship. Since H2X is Acidic Species and HX- is its Salts, it is appearing a Chemical Buffer obeying to HASSELBACH WRITING
pH = pKa1 - log (|H2X| / |HX-|)
Some times later, NaOH added get meaning to Second Chemical Equilibrium : this happen once Chemical Equivalent of Added NaOH moved over half Chemical Equivalent of H2X Acids.
THIS IS LATTER EQUIVALENCE POINT WHICH OBEYS TO ADAPTED HASSELBACH WRITING
pH = pKa2 - log (|HX-| / |X--|)
SINCE NaOH FORMED A SECOND CHEMICAL BUFFER.
This Chemical Buffer has a very low BUFFER CAPACITY BECAUSE EVERY NaOH ADDICTION TO THIS LATTER CONDITION AREN'T ABLE TO NEUTRALIZE ACIDIC SPECIES SO IT AFFECT HEAVILY pH INDEX.
I hope this helps you.
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